1. Field of the Invention
This invention relates to electric fuses of relatively small dimension which are used to interrupt relatively small currents in relatively high voltage circuits.
2. Description of the Prior Art.
Of the many metals and metallic alloys available, a relatively small number possess the requisite properties which make them satisfactory for fabricating the fusible elements to be used in electric fuses of the type used to protect electric circuits. The number becomes even smaller as materials are considered which may be used as the fusible element in fuses of relatively small dimension which are used to interrupt relatively small currents in relatively high voltage circuits. In such applications where the amperages of the fuses are small, the fusible elements are usually made from extremely fine wire-like materials having very small cross sectional areas.
Such extremely fine wire-like elements are so small and fragile that a number of problems are encountered in designing and manufacturing such fuses that are not typically found in the design and manufacture of most electrical fuses. For example, it is customary to fill the casings of such electrical fuses with a granular arc-quenching filler material. However, where the amperages of such fuses are small, the cross sectional area of the fusible elements of the fuses are so small that it is difficult to obtain uniform heat transfer from the fusible elements to the surrounding arc-quenching filler material. For example, if the arc-quenching filler material is a granular material such as quartz sand, there will be minute voids between the grains of the arc-quenching material and the fusible elements, the voids having a lower heat transferring capability than the grains of sand. On the other hand, if the arc-quenching material is a granular material having a finer grain, such as gypsum, the forces which must be used to compact the arc-quenching filler material within the casings of the fuses can displace the fusible elements from their intended positions within the casings, and may even break the fusible elements. Further, even if a fine grain arc-quenching material such as gypsum, could be compacted around the fusible element without breaking the element, the arc-quenching material could develop cracks or voids when the fuse was handled, such cracks or voids having a lower heat transfer capability than the body of filler material.
Furthermore, regardless of whether a large or fine grain arc-quenching filler material is used, the filling of such small fuse casings is an extremely tedious and delicate operation and the vibration and other handling of the fuse associated with achieving a good void-free fill could often result in breakage of the extremely fine wire-like elements in such low amperage fuses.
Consequently, it is desirable to provide low amperage electric fuses which are capable of protecting relatively high voltage circuits which may be provided with extremely short casings, wire-like elements, and that are devoid of granular arc-quenching filler material. One such electric fuse design is disclosed in U.S. Pat. No. 3,460,086 entitled PROTECTORS FOR ELECTRIC CIRCUITS to A. J. Fister.